Sulphide mineral flotation has been practiced since the early 20th century. Its industrial importance is well recognized as the concentrates from flotation can be more economically smelted and refined to provide primary metals. Froth flotation is a process to selectively separate value minerals from waste gangue materials through utilizing the differences in surface hydrophobicity. In general, the flotation process involves the grinding of crushed ore in a dense slurry to liberation size, followed by its conditioning with different reagents in a suitable dilute pulp. The reagents include collectors, depressants, frothers, modifiers, etc. Collectors render the surface of desired minerals hydrophobic by physical/chemical adsorption, which facilitates the attachment of air bubbles that cause the mineral particles to float to the surface of the slurries and form a stabilized froth which is collected for further treatment. Depressants have the reverse action to collectors, causing the surface of undesired mineral particles to become hydrophilic by adsorbing hydrophilic components or by removing the active sites for the collector's adsorption, thus allowing the particles to remain in the tailings fraction. Frothers help to stabilize air bubbles of suitable sizes in the slurry in order to capture and transfer particles to the froth zone. Modifiers are usually used for pH control. The various schemes of froth flotation that are employed are generally quite complex in order to maximize grade and recovery of the valuable minerals present and to maximize rejection of rock and sulphide minerals of little commercial value.
In the processing of sulphide ores for the recovery of non-ferrous pay metals, the common value minerals treated include pentlandite and millerite, chalcopyrite and chalcocite and bornite, galena, and sphalerite for the metals Ni, Cu, Pb and Zn respectively. However, these value minerals are naturally associated with iron sulphides, namely pyrrhotite, pyrite, and marcasite, which have no commercial value and are considered as sulphide gangues. Selective rejection of iron sulphides in flotation can significantly improve the economic value of the concentrate and also reduce the SO2 emissions at smelters where the iron sulphides are significant contributors to these gaseous emissions. However, pyrrhotite rejection is challenging. It not only relates to the abundance of pyrrhotite in the ore, but also to the crystal structure of pyrrhotite (i.e. monoclinic, hexagonal or troilite). Furthermore, pyrrhotite is intimately associated with other minerals, primarily with pentlandite. Selective depression of pyrrhotite without compromising the recoveries of Cu and Ni during flotation is a key to building commercial value in an industrial mineral processing plant.
U.S. Pat. No. 5,074,993 describes a method of flotation of sulphides wherein pyrrhotite is depressed by use of a water-soluble polyamine in an amount >50 g/mt of the ground mineral mixture. The water-soluble polyamine is preferably diethylenetriamine (DETA), and can also be selected from a list that includes triethylenetetramine, tetraethylenepentamine, pentaethylenehexamine, 2-[(2aminoethyl)amino]ethanol, Tris-(2-aminoethyl)amine, N-methyl ethylenediamine and 1,2 diamino 2 methylpropane.
U.S. Pat. No. 5,411,148 describes a process for the improved separation of mono- or multi-metallic sulphide minerals from iron sulphides. The process comprises a conditioning stage before flotation with at least one water-soluble sulphur-containing inorganic compound as a prerequisite step before further conditioning with a nitrogen-containing organic chelating agent which is described in U.S. Pat. No. 5,074,993. The water-soluble sulphur-containing inorganic compound is preferably sodium sulphite (Na2SO3), and can also be selected from the group consisting of sulphides, dithionates, tetrathionates, and sulphur dioxide, in an amount varying from 50 to 600 g/mt of dry solids processed. The nitrogen-containing organic chelating agent is preferably a polyethylenepolyamine such as diethylenetriamine (DETA) used at an adequate dosage for a particular flotation feed. The pyrrhotite is depressed as a result of the combined effects of the sulphur-containing compound and the nitrogen-containing organic compound, added in a particular order.
The aforementioned processes are very effective at increasing Ni and Cu concentrate grade and recovery with selective pyrrhotite depression. However, the use of DETA can complicate the operation of the wastewater treatment regarding the total (soluble and insoluble) Cu and Ni discharged in the effluent. DETA is a strong chemical chelating agent that forms stable complexes with heavy metal ions, such as Cu and Ni. These complexes cannot be precipitated out by raising the pH above 11, as is typically done in the wastewater treatment plants. Instead, a polyamine precipitating agent such as NALMET® 8702 (available from the Nalco Company, Naperville, Ill.) is added to the wastewater to react with the DETA-metal complexes and form a precipitate. However, the precipitates are very fine particles which do not settle in the clarifier, making it difficult to effectively remove the Cu and Ni from the wastewater. In order to avoid a high level of Cu and Ni in the wastewater when using DETA, efforts are being made to identify alternative iron sulphide depressants to reduce or eliminate the use of DETA.
A recent patent from LignoTech (U.S. Pat. No. 8,221,709) describes a method of using hardwood lignosulphonates for separating gangue materials from metallic sulphide ore. The patent specifies three hardwood lignosulphonates obtained from Eucalyptus, Maple, and Birch trees with different sulphur or sulphonate contents and molecular weights, and compared their performances at dosages of ˜250-500 g/mt with NaCN additions in the flotation of a ground ore slurry which comprised copper sulphide, zinc sulphide, or lead sulphide with iron sulphides. The lignosulphonates can be added before or after other reagents and pH adjustments. However, the selectivity between Cu/Ni sulphides and pyrrhotite is not improved with the addition of lignosulphonate alone in the industrial process.
In this sense, the state of the art lacks a method for a) improving the selectivity and recovery in the flotation of Cu/Ni sulphide minerals which are associated with iron sulphides, and b) reducing or eliminating the use of problematic polyamine chemicals (such as DETA) to minimize the negative impact on the environment.